1. Field of the Invention
The present invention relates to semiconductor light emitting devices, and more particularly, to a semiconductor light emitting device that has excellent light extraction efficiency to effectively reflect light moving into the device by increasing the total reflectivity of a reflective layer, and can prevent an increase in operating voltage by maintaining an effective light emitting area and increasing the reflectivity of the reflective layer.
2. Description of the Related Art
In general, semiconductor light emitting devices include semiconductor materials that emit light. For example, light emitting diodes (LEDs) have semiconductor junctions using diodes to convert energy generated by recombination of electrons and holes into light and emit the generated light. The semiconductor light emitting devices are being widely used as lighting, display devices, and light sources. In consideration of energy savings and the protection of environment, the development of semiconductor light emitting devices has been expedited in that they can emit light having desired wavelength with low power consumption and prevent emission of environmental hazardous substances such as mercury.
In particular, the widespread use of cellular phone keypads, side viewers, and camera flashes, which use gallium nitride (GaN)-based light emitting diodes that have been actively developed and widely used in recent years, contributes to the active development of general lighting that uses light emitting diodes. Applications of the light emitting diodes, such as backlight units of large TVs, headlights of cars, and general lighting, have advanced from small portable products to large products having high power, high efficiency, and high reliability. Therefore, there has been a need for light sources having characteristics that satisfy corresponding products.
FIG. 1 is a cross-sectional view illustrating a semiconductor light emitting device 1 according to the related art. In FIG. 1, in the semiconductor light emitting device 1, a reflective electrode 20 is formed on a substrate 10.
The semiconductor light emitting device 1 includes the substrate 10, a first conductivity type semiconductor layer 30, a second conductivity type semiconductor layer 50, and an active layer 40 formed between the first and second conductivity type semiconductor layers 10 30 and 50 to generate light. The first and second conductivity type semiconductor layers 30 and 50 are different conductivity type semiconductor layers. Further, the reflective electrode 20 is provided between the substrate 10 and the first conductivity type semiconductor layer 30.
The semiconductor light emitting device 1, shown in FIG. 1, is designed so that light generated from the active layer 40 is extracted toward the second conductivity type electrode 60. However, when a voltage is applied to the second conductivity type semiconductor layer 50, the first conductivity type semiconductor layer 30, and the active layer 40, and a current flows therethrough, light is generated in an omni-direction at a predetermined point of the active layer 40 and moves.
Therefore, when the light moves in an undesirable direction, for example, if the actually generated light is not extracted to the outside of the semiconductor light emitting device 1 but moves toward the substrate 10, the light is lost inside the semiconductor light emitting device 1 and is not extracted. The reflective electrode 20 is provided so that light moving toward the substrate 10 that is an opposite direction to a light extraction direction is redirected and moves to the light extraction direction.
A vertical electrode-type LED that has a conductive support substrate opposite to the LED and does not have a sapphire substrate is more advantageous than a horizontal electrode-type LED that uses a non-conductive sapphire substrate in terms of electric resistance and heat generation during high-current operation. In order to improve efficiency of the vertical electrode-type LED, an irregular pattern that causes diffused reflection is formed at the surface to increase light extraction efficiency or reflectivity of the reflective electrode is increased to reduce absorption of light by the electrode.
Thus, the reflective electrode 20 is generally formed of Ag or Al that has high reflectivity with respect to visible rays (in particular, blue) and an excellent electrical characteristic to obtain good electric current flow. However, in order to manufacture an LED having high efficiency, the reflectivity needs to be increased more. Further, referring to FIG. 1, the reflective electrode 20 contains Ni 22 as well as Ag 21. Since Ag 21 has weak adhesion to the first conductivity type semiconductor layer 30, Ni 22 is used as an adhesive material. However, when Ni 22 is contained in the reflective electrode 20, Ni 22 absorbs light moving from the active layer 40 to reduce the reflectivity of the reflective electrode 20.
FIG. 2 is a graph illustrating reflectivity according to an incident angle in the reflective electrode of the semiconductor light emitting device, shown in FIG. 1. In FIG. 2, the reflectivity is measured using a reflective electrode included in a blue LED. Referring to FIGS. 1 and 2, it can be seen that the highest reflectivity is obtained when Ag 21 is only used. When Ni 22 is added because of the weak adhesion of Ag 21, the reflectivity according to the incident angle is reduced with increasing content of Ni 22.
Therefore, there has been a need for a method of improving the light extraction efficiency of a semiconductor light emitting device with ease at low cost.